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Lowpass filter specification

`D = fdesign.lowpass`

D = fdesign.lowpass(SPEC)

D = fdesign.lowpass(SPEC,specvalue1,specvalue2,...)

D = fdesign.lowpass(specvalue1,specvalue2,specvalue3,specvalue4)

D = fdesign.lowpass(...,Fs)

D = fdesign.lowpass(...,MAGUNITS)

`D = fdesign.lowpass`

constructs
a lowpass filter specification object `D`

, applying
default values for the default specification option `'Fp,Fst,Ap,Ast'`

.

`D = fdesign.lowpass(SPEC)`

constructs
object `D`

and sets the `Specification`

property
to the entry in `SPEC`

. Entries in `SPEC`

represent
various filter response features, such as the filter order, that govern
the filter design. Valid entries for `SPEC`

are shown
below. The options are not case sensitive.

Specifications options marked with an asterisk require the DSP System Toolbox™ software.

`'Fp,Fst,Ap,Ast'`

(default option)`'N,F3db'`

`'N,F3db,Ap'`

*`'N,F3db,Ap,Ast'`

*`'N,F3db,Ast'`

*`'N,F3db,Fst'`

*`'N,Fc'`

`'N,Fc,Ap,Ast'`

`'N,Fp,Ap'`

`'N,Fp,Ap,Ast'`

`'N,Fp,Fst,Ap'`

*`'N,Fp,F3db'`

*`'N,Fp,Fst'`

`'N,Fp,Fst,Ast'`

*`'N,Fst,Ap,Ast'`

*`'N,Fst,Ast'`

`'Nb,Na,Fp,Fst'`

*

The filter specifications are defined as follows:

`Ap`

— amount of ripple allowed in the pass band in decibels (the default units). Also called Apass.`Ast`

— attenuation in the stop band in decibels (the default units). Also called Astop.`F3db`

— cutoff frequency for the point 3 dB point below the passband value. Specified in normalized frequency units.`Fc`

— cutoff frequency for the point 6 dB point below the passband value. Specified in normalized frequency units.`Fp`

— frequency at the start of the pass band. Specified in normalized frequency units. Also called Fpass.`Fst`

— frequency at the end of the stop band. Specified in normalized frequency units. Also called Fstop.`N`

— filter order.`Na`

and`Nb`

are the order of the denominator and numerator.

Graphically, the filter specifications look similar to those shown in the following figure.

Regions between specification values like `Fp`

and `Fst`

are
transition regions where the filter response is not explicitly defined.

`D = fdesign.lowpass(SPEC,specvalue1,specvalue2,...)`

constructs
an object `D`

and sets the specification values at
construction time using `specvalue1`

, `specvalue2`

,
and so on for all of the specification variables in `SPEC`

.

`D = fdesign.lowpass(specvalue1,specvalue2,specvalue3,specvalue4)`

constructs
an object `D`

with values for the default `Specification`

property `'Fp,Fst,Ap,Ast'`

using
the specifications you provide as input arguments `specvalue1,specvalue2,specvalue3,specvalue4`

.

`D = fdesign.lowpass(...,Fs)`

adds
the argument `Fs`

, specified in Hz to define the
sampling frequency to use. In this case, all frequencies in the specifications
are in Hz as well.

`D = fdesign.lowpass(...,MAGUNITS)`

specifies
the units for any magnitude specification you provide in the input
arguments. `MAGUNITS`

can be one of

`'linear'`

— specify the magnitude in linear units`'dB'`

— specify the magnitude in dB (decibels)`'squared'`

— specify the magnitude in power units

When you omit the `MAGNUNITS`

argument, `fdesign`

assumes
that all magnitudes are in decibels. Note that `fdesign`

stores
all magnitude specifications in decibels (converting to decibels when
necessary) regardless of how you specify the magnitudes.